The present disclosure relates to molded articles that incorporate holograms, more particularly color transmission holograms. Methods of making and using the same are also disclosed.
Holograms are an increasingly popular mechanism for the authentication of genuine articles, whether it is for security purposes or for brand protection. The use of holograms for these purposes is driven primarily by the relative difficulty with which they can be duplicated. Holograms are created by interfering two or more coherent beams of light to create an interference pattern and storing that pattern in a holographic recording medium. Information or imagery can be stored in a hologram by imparting the data or image to one of the two coherent beams prior to their interference. The hologram can be read out by illuminating it with beams matching either of the two original beams used to create the hologram and any data or images stored in the hologram will be displayed. As a result of the complex methods required to record holograms, their use for authentication has been proposed or implemented on articles such as credit cards, software, passports, clothing, electronics and the like.
Two categories of holograms include surface relief structure holograms and volume holograms. Many of the holograms used in security or authentication applications are of the surface relief type, in which the pattern and any data or image contained therein is stored in the structure or deformations imparted to the surface of the recording medium. As a result, the first recorded hologram may be created by the interference of two coherent beams, but duplicates can be created by copying the surface structure using techniques such as embossing. The duplication of holograms is convenient for the mass production of articles such as credit cards or security labels, but it also has the disadvantage that it makes the unauthorized duplication and/or modification of these holograms for use in counterfeit parts possible from the originals using the same mechanism.
Unlike surface holograms, volume holograms are formed in the bulk of a recording medium. Volume holograms have the ability to be multiplexed, storing information at different depths and different angles within the bulk recording material and thus have the ability to store greater amounts of information. In addition, because the pattern which makes up the hologram is embedded, copying cannot be done using the same techniques as for surface relief holograms.
Many types of holographic recording media have been proposed, including silver halide emulsions, photoresists, photopolymers, and the like. Photoreactive dye-based holograms have also been proposed for volume holograms, and they provide advantages not achievable by other holographic recording media, such as processability by thermoplastic techniques (molding, extrusion, lamination, etc.), instant hologram viewability, and latent photosensitivity after the initial writing. However, when incorporated into structures for real-world use, such as for security applications, the latent photosensitivity can lead to degradation of the recorded hologram. While several photostabilization techniques have been proposed, there is still a need for stabilization routes that reduce post-exposure processing requirements such as lamination of further layers onto the holographic structure after exposure.
Thus, there remains a need for improved holographic materials for incorporation into structures such as security or authentication devices.